You've got some company, there were several users posting about excessive heat when i9 is loaded below 20%, there is a good chance they were just failed units, or there were some additional factors - like the dGPU in use when connected to external monitor. Whether i7 in your case will be cooler - I don't know. There are too many depended variables influencing this, whether i9 is binned (like howard2k said, if it is i9 should be cooler), cache organization and hit rates (i9 has larger cache, so it will be hotter, unless somehow it gets higher cache hit rates, which also depends on software used), frequency to power ratio (i9 is faster at least in short bursts, can finish task quicker, but the power requirements to complete this particular task may be higher or lower), fan profiles etc. Essentially you have a set of non-linear equations with a ton of unknown variables. However, from my own experience the higher spec CPU in a given laptop chassis was always hotter than a lower one, and after being burned (literally) by one over-the-top config I always settle for the lowest clocked version of a given chassis, and my 2.2/555x gives me zero heat related issues. I don't really see a reason why i9 would be cooler or less noisy.
But it looks like you're going to be in a driver's seat, you'll have both of them at the same time right? Just run some tests and see for yourself. My only suggestion would be to set the fans to max rpms when comparing, this way you'll eliminate at least the fan profile variable. Even better if you could use Volta and limit the power on both of them to the same level.
If you frequently use external monitor you could look into eGPU, this will remove dGPU completely and give you more breathing room for CPU. Money saved by going to 2.2/555x from 2.9/560x would be enough for a rx580 in a decent eGPU enclosure.
